This application claims the priority of Korean Patent Application No. 2004-103177, filed on Dec. 8, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present relates to a motor control method and, more particularly, to a method of controlling drive speed of a motor operating in an overload condition.
2. Description of Related Art
In general, an air conditioner, which is an example of an air handling unit, is equipped with a compressor motor. The motor is controlled based on a motor drive algorithm so that drive speed of the motor decreases under overload conditions and returns to a normal speed after the overload conditions disappear.
A system employing such an algorithm, such as an air-conditioning system, includes a current transformer (hereinafter referred to as “CT”) incorporated in a power source. The system detects a current flowing in the power source through the CT while a compressor is operating, and controls drive speed of a compressor motor based on the detected current.
A detailed description thereof will now be given with reference to FIG. 1. First, while driving the compressor motor with a predetermined initial speed W (step S1), a motor drive-speed controller decelerates the drive speed by one level (step S5) if a current detected by the CT is greater than a first set current (step S3). The motor drive-speed controller continues to drive the motor with the initial speed W if the current is not greater than the first set current.
After one-level deceleration, the motor drive-speed controller continuously monitors the detected current. The motor drive-speed controller decelerates the drive speed by a further one level (step S7) if the detected current is still greater than the first set current for over a time period of T1 (step S6). Otherwise, the motor drive-speed controller drives the motor with the one-level decelerated speed (step S9).
After a further one-level deceleration, the motor drive-speed controller sets the drive speed back to the initial speed W if the detected current is less than the first set current for over a time period of T2 (step S8). Otherwise, the motor drive-speed controller drives the motor with the decelerated speed.
If the detected current is greater than a second set current (step S4), the motor drive-speed controller brings the motor to a halt (step S10) and then increases the number of times (n) the motor has been halted (step S11). If the number of times (n) reaches a predetermined threshold value within a time period of T4 (step S12), an apparatus equipped with the motor should be completely halted and then an error sign appears on a display (step S13). Otherwise, it is determined whether or not a time period of T3 has elapsed (step S15) while the motor is still halted (step S14). The motor is driven again with the initial speed W if the time period of T3 is determined to have elapsed (step S1). Otherwise, the motor remains in a halt state (step S14).
According to the above-mentioned typical method of controlling a compressor motor, the motor is controlled by accelerating/decelerating drive speed of the motor based on a current detected by the CT.
Accordingly, there is a problem in that it is not possible to operate an air-conditioner compressor if the CT is broken.
Further, if the motor for the air-conditioner compressor is instantaneously accelerated back to the initial speed after it has been decelerated through the above-mentioned two steps S5 and S7 or it has been halted, other adjacent elements composing the air conditioner may be broken down due to over-current which may be instantaneously generated, thereby causing the air conditioner to stop operating.